Clogged vent stack or drain?

When flushing our toilet, the toilet sucks the water out of the trap in the wash basin, but it does consistently flush ok and has not backed up. The other symptom however is that our bathtub drains very slowly - but doesn't appear to be clogged up with anything as it also drains.

Septic system was just recently pumped, it was getting full, but everything looked to be in order.

The only thing I can think of is that maybe the vent stack is clogged (haven't checked yet), but is that consistent with what we're seeing? Could something else be clogged?

Fill up the lav and then pull the stopper . Does the water drain out with a suction? Or does it bounce the water in the toilet some?
Might pe a partially plugged section of the sewer, or something in the vent pipe.
You say the toilet flushes, but I eonder if it flushes like it should.
Run water with a garden hose down the stack on the roof . You should be able to hear the water draining all the way down the line.
If your on the roof have someone flush the toilet. If you hear a glub-glub, then the sewer is slow and needs to be cleaned to the septic tank.

how old is the house? are the stacks open at the top (not capped or screened)? do you have a lot of tall trees?

if it's an older (20+) house in a forest it wouldn't hurt to get the stacks flushed. you can often do it yourself if you have a long hose and adequate pressure - and you're not averse to walkin' around on your roof. otherwise get a plumber. and then put a cap or screen on dem stack openings.